1. Field of the Invention
The present invention relates to a method for pulling a silicon monocrystal from a melt, comprising the pulling of one conical portion in each case at the beginning and at the end of the monocrystal and the pulling of a cylindrical portion between the conical portions. In addition, the invention also relates to an apparatus for carrying out the method.
2. The Prior Art
The invention is an improvement in the method known by the name "Czochralski method" for producing monocrystals. According to this method, a seed crystal which has previously been immersed in a melt is pulled away from the melt surface at a defined speed, such that an ingot-shaped monocrystal grows on the lower side of the seed crystal. After a so-called thin neck has been pulled, the diameter of the growing monocrystal is first increased so that a conical portion, the so-called initial cone, is produced. After the initial cone, the diameter of the growing monocrystal is kept constant and a cylindrical portion is pulled. Finally, the diameter of the monocrystal is reduced again. The portion produced thereby at the end of the monocrystal is referred to as the end cone. Changes in the monocrystal diameter are substantially due to changes in the pulling speed and to changes in the temperature conditions in the melt, in particular in the region of the crystallization boundary.
In the past, it was usual to cut away the initial cone, to cut away a 30 to 50 mm long portion of the cylindrical portion adjacent to the initial cone, and to cut away the final cone from the monocrystal. Then only the cylindrical main part of the monocrystal could be cut into semiconductor wafers. In the case of relatively short monocrystals having a relatively large diameter in the cylindrical portion, this procedure meant discarding an appreciable part of the potentially usable crystal material.
If that part of the cylindrical portion which adjoins the initial cone is nevertheless cut into semiconductor wafers, an annular region (stacking fault ring) in which oxidation-induced stacking faults occur in high density can be detected on the wafers. The manufacturers of electronic components prefer, however, semiconductor wafers with as low a stacking fault density as possible. Semiconductor wafers having a stacking fault ring are therefore considered inferior.
It is now known that the diameter of the stacking fault ring is proportional to the pulling speed with which the monocrystal was pulled, and in particular, at the instant in time when the semiconductor material which comprises the semiconductor wafer produced later crystallized. At a certain pulling speed, the stacking fault ring disappears, as it were, because its diameter coincides with the diameter of the future semiconductor wafer. An attempt is therefore made to reach this pulling speed quickly during the pulling of the monocrystal in order to limit the occurrence of the stacking fault ring to the initial cone and to as short a part as possible of the adjacent cylindrical portion. This is achieved, however, only to an inadequate extent. In addition, it is known that the breakdown strength of oxide films, which is referred to in specialist jargon as gate-oxide integrity (GOI) and is an important criterion in assessing the quality of semiconductor wafers, is dependent on the pulling speed in an indirectly proportional manner.